Highly loaded workpieces, such as for example turbine blades intended for turbines, in particular gas turbines, such as moving and stationary blades, are coated to improve their temperature resistance and/or abrasion resistance with metals, metal alloys or ceramics suitable for the purpose. The coating takes place by means of a spray coating apparatus, in which the turbine blade is spray-coated. Examples of spray coating methods are atmospheric plasma spraying (APS) and high velocity oxygen fuel spraying (HVOF).
In the case of turbine blades, only those surfaces that are exposed to the medium flowing through—in the case of a gas turbine hot gas—are coated, while the blade root—root body or root plate—is covered by a mounting for fixing the turbine blade during the coating operation. For this purpose, the mounting has a cavity with an opening into which the turbine blade is inserted with its blade root, so that it protrudes with the portion of the blade that is to be coated from the opening of the cavity. The cavity may additionally have a connection to a compressed air source, from which compressed air can be blown into the cavity for cooling during the coating operation.
After the turbine blade is inserted into the cavity of the mounting, a gap remains between the inner side of the mounting and the turbine blade. This gap is filled by a shielding in the form of shielding plates. In order to avoid bridging of the coating material between the turbine blade and the shielding plates and resultant flaking off of coating material from the turbine blade when it is removed from the mounting, said plates are inserted in clamping devices in such a way that they protrude from the opening while forming a stepped transition from the turbine blade to the mounting. However, it must be accepted here that regions of the workpiece that protrude from the mounting and according to the specification must remain free of coating because they have already been mechanically brought to their final dimensions are spray-coated. This so-called “overspray” must therefore be ground away after the spray coating in a subsequent operation (overspray grinding). This additional method step is time-consuming and costly.
DE 698 15 644 T2 discloses a spray coating apparatus with a mounting into which a multiplicity of turbine blades can be inserted. Since only the tips of the turbine blades are to be coated, the blade region is provided with a plate covering which leaves only the tip of the blade exposed. The turbine blade is held by means of a block of an elastic material, which closely surrounds a part of the blade region that is adjacent the blade root in such a way that a compressive force is exerted for the purpose of fixing the shielding plate. The block is secured within the cavity of the mounting. In DE 698 15 644, reference is also made to the use of aluminum foil tape for the purpose of covering blade regions. The mounting for the spray coating apparatus described above is not suitable for coating the entire surface area of the blade region of a turbine blade, since a considerable part of the blade region is covered by the mounting itself.